The alignment of symmetrical molecules can be achieved through a crystal packing relationship so long as the molecules are symmetrical. For example, Chen et al, Stereochemistry and Stereoelectronics of Azines. A Solid State Study of Symmetrical, (E,E)-Configured, Para-Substituted (H, F, Cl, Br, CN) Acetophenone Azines, The Journal of Organic Chemistry, 59 (1994) (hereinafter Chen I) and Chen et al, Polymorphism and C.dbd.N--N.dbd.C Conformational Isomers of Azines: X-ray Crystal and Ab Initio Structures of Two Rotational Isomers of Methyl (para-Tolyl) Ketone Azine, "Stereochemistry and Stereoelectronics of Azines, Part 3", Angwandte Chemie (English Reprint 1994/33), Angew, Chem. Int. Ed. Engl., Vol. 33, No. 10 (1994) (hereinafter Chen II), disclose the stereochemical configurations of para-disubstituted symmetrical acetophenone azines and various crystal packing schemes associated with these symmetrical molecules. In Chen I, five symmetrical acetophenone azines are disclosed. These include the basic substituted compound, 1,4-diphenyl-1,4-dimethyl-2,3-diazabutadiene, and the corresponding di-fluoro, di-chloro, di-bromo and di-cyano compounds with the functional substituents in each case in the para position with respect to the azinyl bridge. Chen II discloses the corresponding di-methyl compound, the methyl groups again being in the para position.
Chen I discusses crystallographic data and crystal packing effects for the five compounds disclosed there. The unsubstituted and di-fluoro, di-chloro and di-cyano phenyladiene azines all exhibit monoclinic symmetry while the di-bromo structure has orthorhombic symmetry. As disclosed in Chen II, the unsubstituted compound and the di-halo substituted compounds all exhibited gauche conformation with torsion angles that decrease with the increasing electronegativity of the halogen substituents. The di-cyano compound is described as being in a trans N--N conformation in which the torsion angle is 180.degree., notwithstanding an electronegativity of the cyano group between the values for chlorine and bromine.
The crystal structures and packing configurations of additional para-disubstituted symmetrical azines are disclosed in Glaser et al, Comparative analysis of crystal structures of E,E-configured para-substituted acetophenone azines with halogen, oxygen, nitrogen and carbon functional groups, J. Chem. Soc. Trans. 2, pp. 1449-1458 (1995). Here, the solid state structures of fifteen para-disubstituted azines, including those disclosed in Chen I and Chen II are examined. Additional substituent groups, in addition to those discussed in the Chen et al publications include amino and alkylamino groups, ethers and fluorinated ethers, esters, nitro groups and amide groups.
In contrast to the symmetrical molecules disclosed in the aforementioned Chen et al and Glaser et al papers, asymmetrical molecules can be employed to produce materials showing non-linear optical (NLO) effects. These materials include aromatic materials either bridged or unbridged which are substituted with electron donor and electron acceptor substituents. For example, Eaton, D. F., Nonlinear Optical Materials, Articles Science, Vol. 253, pp. 281-287 (July 1991) discloses second order NLO materials based on organic derivatives such as 4-Bromo-4'-methoxychalcone (BMC) and 3-Methyl-4-methoxy-4'-nitrostilbene (MMONS), the latter said to have a high second harmonic generation (SHG) value. Similar such materials are disclosed in Williams, D. J., Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities, Agnew. Chem. Int. Ed. Engl., Vol 23, pp. 690-703 (1984) which discloses aromatic groups substituted with electron acceptor and electron donor groups which may be in the ortho-, meta- or para position as well as bridged structures in which the bridges take the form of ethylene or polyethylene bridges or aminoethylene bridges. Somewhat similar bridged materials are disclosed in Long, N. J., Organometallic Compounds for Nonlinear Optics--The Search for En-light-enment!, Agnew. Chem. Int. Ed. Engl., Vol 34, pp. 21-38 (1995) in which the bridges may take the form of silane or polysilane groups as well as polygermanes and various metallocene compounds.
The difficulty with such materials heretofore has been that the materials having molecular dipole moments have been thought to invariably orient in their crystalline structures in a manner so as to produce little or no macroscopic dipole moments. In view of the difficulties encountered with either organic or inorganic crystalline materials, it has been proposed to form electric field aligned NLO chromophores in polymeric materials. Such polymeric materials are configured with spaced electron donor-electron acceptor groups in polymeric matrices which can be "poled" by an applied electrical field in order to align the polar electron donor acceptor groups in a manner to impart macroscopic NLO properties to the polymer. By way of example, U.S. Pat. No. 4,810,338 to DeMartino et al discloses liquid crystalline NLO polymers in which a polymer mainchain, such as polyvinyl acrylate, supports side chain connected electron donating or electron withdrawing groups. Suitable electron donor groups disclosed in DeMartino are amino, alkyl, alkoxy, alkylthio, hydroxy, thiolo, acyloxy, vinyl, halo and the like. Suitable electron-withdrawing substituents include nitro, haloalkyl, cyano, acyl, alkanoyloxy, alkoxysulfonyl, and the like. A macroscopic dipole moment can be imparted to the polymer structure by means of a "poling" procedure in which an electric field is applied to induce a nonconcentrosymmetric orientation of the side chain NLO moieties. As disclosed in DeMartino, the liquid crystalline polymer melt is disposed within a "poling" cell in an applied electrical field is maintained for a 24 hour period in which the polymer is cooled to obtain the noncentrosymmetrically oriented polymer matrix. The non-linear optical moieties are said to be aligned parallel to the direction of the electric field.
Other polymer structures having NLO side chains are disclosed in U.S. Pat. No. 5,224,196 to Chanarian et al. Here, non-linear optical moieties incorporated into the polymer matrix include various electron donor and electron acceptor groups linked via conjugated units such as azol linked or alkylene linked spacer groups. As disclosed here and also in U.S. Pat. No. 5,204,178 to Licht and U.S. Pat. No. 5,401,612 to Etzbach et al which disclose polyesters or other polymers containing NLO chromophores, the polymers can be subjected to a poling operation in order to arrive at macroscopic dipole moments.